1. Field of the Invention
The present invention generally relates to a high-performance projection display system using at least two reflective light valves and a light splitting and combining device, typically a polarizing beam splitter, and more particularly to a projection display system which uses a wavelength-selective retarder device to project high contrast images from reflective light valves to and from which lights go through two faces of the light splitting and combining device.
2. Discussion of Related Art
Reflective liquid crystal on silicon (LCOS) imagers are expected to become the lowest cost light valve (LV) technology for such applications as high definition television (HDTV), advanced television (ADTV), and monitors of medium-to-large diagonal about 28 inches or more. Though mature LCOS technology is expected to be cheaper than competing digital micro mirror device (DMD) and polysilicon technologies, its adoption is hampered at present by limitations which its reflective mode of operation imposes on projection optics.
These limitations may be understood as follows: A light which is unchanged in direction or polarization upon reflection from a reflective LV will necessarily retrace its path back to the light source due to the intrinsic reversibility of efficient passive optical elements. A light that remains in the illumination state is thus unable to contribute to the image. Each reflective light valve must therefore be illuminated in a dark-state light, and space must be provided above the light valve to separate the light which the LV converts to a bright state from the dark state. With LCOS light valves this separation is most commonly accomplished by a polarizing beam splitter (PBS), as shown in FIG. 1. An illumination light supplied by a light source 10 enters one face (i.e. one port or channel) 12a of the PBS 12 and is redirected by a polarizer-coated PBS hypotenuse 14 (for example in reflection as shown in FIG. 1) to a second channel 12b of the PBS, where it illuminates one or more reflective light valves 16. The bright-state light returned from these light valves is then passed by the hypotenuse coating 14 (for example in transmission) to exit the PBS through a third face 12c. A dark-state returned light is reflected by the hypotenuse coating 14 and returns to the source 16.
One problem with the PBS arrangement shown in FIG. 1 is that it forces separation of the light valve from a projection lens, so that a more complex lens is required for high-quality imaging. It is therefore desirable to minimize the space impact of the PBS. Moreover, when the projector employs multiple light valves, it is desirable to deploy the associated PBS""s as efficiently as possible. A particular inefficiency of the arrangement shown in FIG. 1 is that only three of the four available PBS faces or ports are used. This in turn increases the complexity of a system that employs the subsystem of FIG. 1 to project color images from multiple light valves. For example, FIG. 2 shows a prior art arrangement in which three PBS""s 20, 21, 22 associated with three Lvs 23, 24, 25 (one for each of the red [R], green [G], and blue [B] colors) are integrated into a projection display system using a number of color separating and combining dichroics 26, 27, 28. Each LV has a dedicated PBS, and the layout becomes fairly complex because the separating and combining dichroics 26, 27, 28 must essentially wrap the light around these PBS""s 20, 21, 22. In practice, additional lenses (not shown) must be added to accommodate the differing lengths of the various paths taken by the light.
A different example is shown in FIG. 3, comprising a prior art optical system in which R, G, and B lights are alternately delivered to a single light valve 30 by using a light wheel 32 positioned between a light source 34 and a PBS 36. Though the system of FIG. 3 is simple optically, the light valve 30 is not used in an efficient way because only one color is projected at a given time. Alternative versions of the FIG. 3 system use, for example, a telecentric pair of micro-lens arrays, or scanning illumination optics, to simultaneously deliver the R, G, and B illumination bands to spatially separated sub-pixels in the light valve. Here the inefficiency manifests itself in the constraint that only one-third of light valve area can be made available for each color.
U.S. Pat. No. 5,517,340 (hereinafter the ""340 patent), issued on May 14, 1996 to Doany et al. and hereby incorporated by reference, discloses a more efficient use of the PBS than the arrangement of FIG. 3. As shown in FIG. 4, which is a reproduction of FIG. 6 of the ""340 patent, one can make use of two channels of a PBS to illuminate light valves. The embodiment shown in FIG. 4 uses a xe2x80x9csquirrel-cagexe2x80x9d configuration for the color wheel, rather than the disk shown in FIG. 3. In the arrangement of FIG. 4, a green light is always introduced in P polarization state. In other words, for green wavelengths the incident electric field is always polarized within the plane of the figure. The green light is thus directed through the PBS hypotenuse to illuminate the light valve at the right face of the PBS which is dedicated to green image information. Red and blue lights are always introduced in S polarization state. That is, red and blue lights are polarized perpendicular to the plane of incidence. More precisely the S polarized component of the illumination light is alternately switched in color between red and blue. Red and blue lights are therefore reflected by the PBS coating to the light valve at the top face of the PBS, which is alternately loaded with red and blue image information. Or, the red/blue illumination and the loading of red/blue image information can be scrolled along the light valve in a synchronous fashion.
The goal of the system shown in FIG. 4 is to encode one set of colors or wavelengths of the illumination light in P polarization, in order that the PBS hypotenuse coating illuminates one LV in transmission with these colors, while encoding a second set of colors or wavelengths in S polarization, in order to illuminate a second LV in reflection. This assumes a PBS of the conventional kind where the hypotenuse coating reflects S polarization and transmits P.
However, a problem with the system of FIG. 4 is that when illuminating light is returned from the green light valve in dark state, the hypotenuse coating of the PBS will inevitably reflect several percent of the dark state light out the bottom exit face of the PBS and onward to a projection screen. This is because a practical hypotenuse coating of the PBS is not capable of entirely transmitting back through to the source the green light that remains in the dark state upon reflection from the green LV. Available hypotenuse coatings will instead have a residual reflectivity in P polarization of at least a few percent averaged over practical angular ranges. Thus, the image will contain non-negligible residual green intensity even when the green light valve is in the black state, and contrast of the image will be degraded accordingly. The PBS also directs unwanted P-polarized green light onto the red/blue LV, further contributing to the residual dark-state intensity. One might contemplate putting a cleanup polarizer in the bottom output face to reduce the residual dark-state P polarization while passing bright-state S polarization, but unfortunately these polarization assignments are reversed for the red and blue lights. That is, for red and blue colors or wavelengths, P polarization is the bright state and thus cannot be reduced. A PBS operating in transmission can usually by itself adequately reduce the residual S-polarized light without a cleanup polarizer. Thus in the layout of FIG. 4 high contrast can be provided in red and blue if the input red and blue lights are purely S polarized, but the problem of low green contrast remains.
The ""340 patent describes one method for improving contrast in the optical system of FIG. 4. In accordance with the improved method, a light valve dedicated to red and green is alternately illuminated in reflection with S-polarized red and green illumination, while a blue light valve is illuminated in transmission with P-polarized blue light; a color filter is positioned in front of the red/green light valve to screen it from mis-reflected P-polarized blue illumination. However, this method still provides poor contrast from the blue light valve, due to the aforementioned limitations of the PBS in filtering the dark state P polarization in reflection.
Another solution is to use a total-internal-reflection (TIR) prism and oblique illumination instead of a PBS, as disclosed in the U.S. patent application Ser. No. 085,065 entitled xe2x80x9cLightvalve Projection System in which red, green and blue image subpixels are projected from two light valves and recombined using total reflection prismsxe2x80x9d filed on May 27, 1998. This, however, requires that the solid angle of the projection lens be doubled (if rotationally symmetric), that the illumination and collection optics be corrected for a semicircular pupil, and in addition that the hypotenuse coating be phase corrected to avoid compound angle depolarization. This coating must also be designed to minimize the dead band imposed by the impossibility of achieving a perfectly sharp transition between angles of near-unity transmittance and the TIR regime. In other words, at angles sufficiently close to critical, the coating can no longer provide good antireflection.
International patent application NO. WO9707418 entitled xe2x80x9cThin Film Polarizing Devicexe2x80x9d and published on Feb. 27, 1997, discloses a PBS element based on frustrated total reflection that can provide reasonably good contrast in both S and P polarizations. However, this element operates at a very steep angle of incidence, about 70 degrees incidence at the hypotenuse, making the PBS element large in at least one axis, departing quite far from a cubic shape, which ultimately imposes on the illumination and projection lenses about a doubling of the optical working distance, significantly increasing cost. Also, the extinction ratio provided by these beam splitters, though better than that of conventional PBS cubes used in two channels, is insufficient for some applications.
Accordingly, it is an object of the present invention to provide a projection display system having a PBS and using at least one light valve positioned at a face of the PBS for a set of wavelengths and another at least one light valve at another face for another set, without sacrificing contrast in any color.
In accordance with the teachings herein, the present invention provides a projection display system which includes a light source adapted to supply an illumination light having a common polarization at first and second sets of wavelengths. A wavelength-selective retarder device is positioned to receive the illumination light from the light source and to produce a first dark-state light having a first polarization state at the first set of wavelengths and a second dark-state light having a second polarization state at the second set of wavelengths. A first reflective light valve device is adapted to receive the first dark-state light and to produce a first bright-state light by rotating the polarization from the first polarization state to the second polarization state. A second reflective light valve device is adapted to receive the second dark-state light and to produce a second bright-state light by rotating the polarization from the second polarization state to the first polarization state. A light splitting and combining device is positioned between the wavelength-selective retarder device and the first and second reflective light valve devices. The light splitting and combining device is adapted to receive the first and second dark-state lights from the wavelength-selective retarder device and to direct the first and second dark-state lights to the first and second reflective light valve devices respectively, and is also adapted to receive the first and second bright-state lights reflected from the first and second reflective light valve devices respectively and to direct the first and second bright-state lights to a screen. A wavelength-selective filtering device is positioned to receive the first and second bright-state lights from the light splitting and combining device and to substantially reduce a residual dark-state light. An image light to be projected onto the screen is thus produced.
In one embodiment of the present invention, the wavelength-selective retarder device comprises a retarder stack adapted to rotate the polarization of the illumination light at either the first or second sets of wavelengths by 90 degrees.
In another embodiment of the present invention, the wavelength-selective filtering device comprises a wavelength-selective retarder adapted to rotate the polarization of either the first or second bright-state light by 90 degrees so that the first and second bright-state lights have a common polarization, and a linear polarizer adapted to substantially eliminate a light having a polarization orthogonal to the common polarization.
In another embodiment of the present invention, the wavelength-selective filtering device comprises a first wavelength-selective polarizer adapted to block a light having the first polarization state at the second set of wavelengths while passing other lights having a polarization different from the first polarization or having a wavelength out of the second set of wavelengths. In this embodiment, the wavelength-selective filtering device may further comprise a second wavelength-selective polarizer adapted to block a light having the second polarization state at the first set of wavelengths while passing other lights having a polarization different from the second polarization or having a wavelength out of the first set of wavelengths.
In a further embodiment of the present invention, the wavelength-selective retarder device comprises a retarder stack adapted to rotate the polarization of the illumination light at either the first or second sets of wavelengths by 90 degrees. The retarder stack is followed by a wavelength-selective polarizer adapted to block a light having the second polarization at the first set of wavelengths. In turn, the wavelength-selective filtering device comprises a wavelength-selective polarizer adapted to block a light having the second polarization at the second set of wavelengths. The wavelength-selective polarizer is followed by a combination of a retarder stack adapted to rotate the polarization of either the first or second bright-state light by 90 degrees so that the first and second bright-state lights have a common polarization, and a linear polarizer adapted to substantially eliminate a light having a polarization orthogonal to the common polarization.
In a still another embodiment of the present invention, a first color filter is positioned between the light splitting and combining device and the first reflective light valve device. The first color filter is adapted to absorb a light having a wavelength within the second set of wavelengths while passing a light having a wavelength within the first set of wavelengths. A second color filter is positioned between the light splitting and combining device and the second reflective light valve device. The second filter is adapted to absorb a light having a wavelength within the first set of wavelengths while passing a light having a wavelength within the second set of wavelengths. In another embodiment of the present invention, the assignment of the first and second sets of wavelengths to the first and second reflective light valve devices is switched sequentially in time.
In a further embodiment of the present invention, the wavelength-selective retarder device comprises a tilted dichroic mirror, a tilted quarterwave retarder stacked over the tilted dichroic mirror, and a tilted mirror stacked over the tilted quarterwave retarder. The wavelength-selective retarder device may further comprise an untilted wavelength-selective linear polarizer positioned between the tilted dichroic mirror and the polarized beam splitter.
In all embodiments described above, the light splitting and combining device may have a first face adapted to receive the illumination light from the light source, a second face adapted to output the first and second bright-state lights, a third face adapted to output the first dark-state light directed to the first reflective light valve device and to receive the first bright-state light reflected from the first reflective light valve device, and a fourth face adapted to output the second dark-state light directed to the second reflective light valve device and to receive the first bright-state light reflected from the second reflective light valve device.
It is noted, in all preceding embodiments, at least either one of said first and second reflective light valve devices may include a plurality of reflective light valves.